Newcastle disease (ND), also referred to as Asian fowl plague, is a highly infectious devastating disease caused by NDV. With high morbidity and mortality rates, ND is listed as one of the two class A poultry diseases (the other one is avian influenza) by the Office International des Epizooties (OIE) and is a national mandatory immune disease. First breaking out in Java, Indonesia and Newcastle, England in 1926, ND among chickens has always been prevalent around the world except the Oceania, and has caused tremendous economic losses in the world. In China, ND is also one of the most destructive poultry diseases, and the disease has been prevalent in many regions in China. ND is mainly controlled through immunization and prevention. In recent years, with wide application of vaccines, large-scale outbreaks and prevalence of ND have been significantly controlled. However, due to incomplete vaccine cold chain system, inappropriate usage methods, and unscientific immunization procedures, typical and atypical NDs constantly emerge. Constant enlargement of the range of ND hosts and emergence and prevalence of mutated strains make prevention and control of ND even more complicated.
Currently, ND vaccines internationally produced and used are classified into two types, that is, live vaccines and inactivated vaccines. The live vaccines include low virulent strain vaccines and moderate virulent strain vaccines. The low virulent strain vaccines include lineage II vaccine (B1), lineage III vaccine (LaSota strain), clone 30, V4, etc., and the moderate virulent strain vaccines include lineage I vaccine, Roskin strain, Komorov strain, Hert 33 strain, Mukteswar strain, etc. Some low virulent live vaccines have a unique heat-resistant feature and are known as heat-resistant live vaccines, and representative strains thereof include V4, 1-2, HB92, and TS09-C strains. This type of vaccines has advantages of being heat-resistant, low virulent, highly effective in immunization, infectious within the group, and capable of performing immunization by multiple approaches (food mixing and spraying), and is applicable to prevent and control ND among various poultries such as chickens, pigeons, and quails. Compared with other non-heat-resistant vaccines, the vaccines are advantageous in southern areas with generally high temperatures and rural areas with poor cold chain conditions, and play an important role in preventing and controlling of the ND.
With the rapid development of molecular biology, basic molecular research on NDV progresses greatly. Genomic RNA of NDV, together with nucleoprotein (NP), phosphoprotein (P), and large polymerase protein (L) coded by the genomic RNA, form a nucleoprotein complex, which subsequently starts a first round of transcription of the RNA and translation and synthesis of virus protein. Components of the virus produce infectious progeny viruses through self-packaging. According to this principle, European scholars established the first reverse genetic manipulation system of highly pathogenic NDVs in 1999. Studies shown that exogenous reporter genes or immunogenic genes can be inserted at different sites of an NDV genome and expressed, and the NDV genome still maintains a high level of genetic and expression stability after many times of serial passage in cells or chick embryos.
Using attenuated heat-resistant NDV strains as live virus vaccine vectors, among other things, has the following prominent advantages. (1). The attenuated heat-resistant NDV strains can be preserved and transported at room temperature, which reduces dependence on the cold chain system, and the strains are more suitable for using in high temperature areas. (2). The attenuated heat-resistant NDV strains have a high group infection rate and a better immunization effect. (3). The vaccine thereof has extremely low toxicity and is not lethal to chick embryos, so that chick embryo immunization or zero-day immunization can be performed. (4). A replication process is from RNA to RNA without a DNA stage and with no possibility of integration with cell genomes. (5). Humoral immunization, mucosal immunization, and cell immunization can be induced at the same time, which produces more comprehensive immunization protection. (6). The vaccine can be provided in multiple manners, such as food mixing, water supplying, and spraying, thus is convenient to use. (7). Attenuated NDV strains have high-titer chick embryo growth and low growth cost. It is required in China to immunize newborn chicks with attenuated NDV vaccines, and at least more than one billion doses of attenuated vaccines are used each year for preventing and controlling ND. Therefore, using the attenuated heat-resistant NDV strains as live virus vaccine vectors provide tremendous economic significance.
Since the first heat-resistant NDV strain, the V4 strain, is isolated in 1966, many heat-resistant NDV strains have been obtained through breeding and isolation, for example, 1-2, HB92, and TS09-C strains. Some research groups attempted to transform heat-resistant strains into heat-resistant vectors, however, no successful research has been reported. For example, Jiang (Jiang, Yanlong, “Sequencing of whole genome of NDV V4 strain and the construction of cDNA clone,” Northeast Agriculture University, doctoral dissertation, 2010) reported the construction of a transcription plasmid and a helper plasmid of a heat-resistant V4 strain, but fails to rescue a recombinant heat-resistant V4 strain. Further, Jiang (Jiang et al., “Plasmids driven minigenome rescue system for Newcastle disease virus V4 strain,” Mol Biol Rep, 2009, Vol. 36(7), pp. 1909-1914) discloses the construction of a transcription plasmid and a helper plasmid of minigenomes of a heat-resistant V4 strain, which implements expression of an exogenous gene in cells, but a recombinant heat-resistant virus still could not be obtained.
Among numerous references about ND live vaccines, no ND live vaccine vector having a heat-resistant characteristic is reported. For example, Chinese Patent Application No. 200510097997.8, entitled “Newcastle Disease LaSota Vaccine Strain Reverse Genetic Manipulation System and Use Thereof,” discloses a non-heat-resistant live vaccine vector based on an NDV LaSota vaccine strain, but the application does not concern a heat-resistant live vaccine vector. Chinese Patent Application No. 200610075781.6, entitled “Recombinant Attenuated Newcastle Disease LaSota Vaccine Strain Expressing HA Protein of Avian Influenza Virus H5 Subtype,” discloses an avian influenza-ND bivalent genetic engineering live vaccine constructed by using an ND LaSota vaccine strain vector, but the application does not concern heat-resistant live vaccine vector.
Therefore, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies.